The long-term objectives of these studies are: (1) to clarify the roles of various enzymatic pathways in cysteine metabolism in mammalian tissues, specifically to determine the enzyme systems involved in the desulfhydration of cysteine, the oxidation of cysteine to taurine, and the mitochondrial oxidation of cysteine, and (2) to determine the changes in cysteine metabolism that occur during development and aging. Specific studies will include: (1) an investigation of the relative roles of cysteine aminotransferase plus 3-mercaptopyruvate sulfurtransferase, cystathionine gamma-lyase, and cystathionine beta-synthase in the conversion of cyst(e)ine sulfur to sulfide, (2) the determination of the roles of cysteine oxidation via cysteine sulfinic acid or by 4'-phosphopantetheine and cysteamine in taurine production, (3) the determination of the role of mitochondrial enzymes in the oxidation of cysteine and the metabolic pathway(s) involved in the oxidation, and (4) a study of the changes in the activities of cysteine-degrading enzymes and of in vivo cysteine degradation during development and aging. Both in vitro and in vivo studies will be carried out using both the rat and the cat. Tissues that will be used for the in vitro experiments will include liver, kidney, brain, and heart.